Various transmission services, e.g., high definition television ("HDTV") services, currently use or are expected to use a Quadrature Amplitude Modulated ("QAM") signal for the transmission of data. The data transmitted by such services, in the form of a QAM analog transmission signal, represents a bitstream comprising a series of symbols, each symbol comprising a number of bits.
To recover the data from a QAM or other signal comprising a series of symbols, a receiver is required to convert the analog signal received into a digital signal and to distinguish between the individual symbols which comprise the signal bitstream. Symbol timing recovery is used to recover a clock signal at the symbol rate, or a multiple of the symbol rate, from the modulated waveform that is received. This clock signal may then be used by the receiver to convert the continuous-time received signal into a discrete-time sequence of data symbols.
QAM demodulators usually rely on symbol timing recovery before any other form of synchronization. Symbol timing, also known as symbol synchronization or bit synchronization, is required for correct sampling of a received QAM analog waveform to minimize inter-symbol interference ("ISI") distortion and to establish a baseline for symbol timing.
Numerous techniques presently exist for symbol timing recovery, such as spectral line extraction and use of least mean square ("LMS") timing circuits. Currently, the spectral line extraction technique is the most popular method of symbol timing recovery and is frequently implemented using analog circuits.
Known timing recovery circuits commonly use analog timing recovery techniques. Such timing recovery circuits may pass a received QAM signal through diodes and then use the envelope of the QAM signal to extract bit timing information. Such known analog circuits have several disadvantages over digital timing recovery circuits. One disadvantage is that such analog circuits fail to make use of the digital matched filtering principle of Nyquist filters which is important when the transmitted signal has only small amounts of excess bandwidth for pulse shaping.
In the case of HDTV signals which may have only 10 percent excess bandwidth for pulse shaping, the resulting tone after a non-linear operation has a very low signal to noise ratio ("SNR"). In such a case, the SNR improvement which can be obtained using a matched Nyquist filter as opposed to an analog filter becomes important since it may result in less clock jitter in the timing signal which is recovered from the QAM signal than would be possible without the use of a matched Nyquist filter.
Known analog timing recovery circuits also have the disadvantage of requiring highly accurate analog phase lock loop circuits which can be difficult and costly to implement. Still another disadvantage of analog timing recovery circuits is that they are difficult and expensive to implement as part of a chip set.
The difficulty and relatively high cost of implementing analog timing recovery circuits is due in part to the sensitivity of component tolerance mismatching which makes it necessary to fine tune analog timing recovery circuits. Such fine tuning is often time consuming and thus adds greatly to the cost of an analog timing recovery circuit. Accordingly, analog symbol timing recovery circuits may be both costly and difficult to implement.